In conventional wire electrical discharge machining apparatuses, the machining conditions are adjusted such that the machining energy is increased in order to improve the machining speed. However, when the machining energy is increased, the wire electrode becomes easy to break. Moreover, the wire becomes easy to break due to the aging of the components of the wire travelling system. Furthermore, when a workpiece is machined into a shape that tends to make machining unstable, the wire also becomes easy to break. If the machining power supply and wire traveling are not stopped immediately after the wire breaks, electrical discharge occurs at places other than at the workpiece, which is dangerous. Consequently, it is necessary to mount wire breakage detecting devices on wire electrical discharge machining apparatuses.
Wire breakage detecting devices include one that uses a method of detecting a wire breakage from the difference in the rotational speed between the rotational components on the wire traveling path (for example, see Patent Literature 1). With this method, the rotational speed is detected at any two of the wire supplying bobbin around which the wire electrode is wound, the tension roller that applies tension to the wire electrode paid out from the bobbin, the collecting roller that collects the wire electrode at a predetermined speed, and the guide pulley that stabilizes the wire electrode as it travels and changes the traveling direction, and then, when the difference in the rotational speed exceeds a predetermined value, it is detected that a breakage has occurred.